We propose to study the molecular basis of sorting of apical membrane proteins in polarized epithelial cells. We will do this by comparing and contrasting how two different epithelial cells types--the MDCK (kidney) and the WIF-B (hepatocyte) cells-- sequester and package two apical ectoenzymes. The first is dipeptidyl peptidase IV (DPPIV), belonging to the class of single transmembrane (Type II) proteins; and the second is 5' nucleotidase (5'NT), a glycolipid-anchored protein. Hepatocytes use an indirect, basolateral-to-apical transcytotic pathway(s) to deliver these proteins, while MDCK cells deliver them directly from the trans-Golgi network (TGN). However, when MDCK cells recover from prolonged treatment with the drug Brefeldin A, the retrieve missorted DPPIV and 5'NT from the basolateral surface and subsequently deliver them to the apical surface (ie, use the indirect route). The polymeric IgA-receptor (pIgA-R) normally uses a transcytotic pathway in both cell types. Do these proteins and cell types share a common (indirect) pathway? How similar or different are the indirect versus direct mechanisms for apical membrane targeting?